Piston-pin bearing for internal combustion engines

ABSTRACT

The invention relates to a piston-pin bearing for an internal combustion engine wherein a cylindrical piston pin ( 8 ) is mounted. The inventive piston-pin bearing is provided with an improved form of hub bore in comparison with prior art, significantly reducing mechanical stress in said piston and therefore resulting in a longer service life for said piston. Said form also avoids the formation of noise in the piston-pin bearing. According to the invention, this is achieved by providing the lateral surface ( 5 ) of the hub bore ( 2 ) with a highly oval shape along the entire axis of the hub in the equatorial-zenith-equatorial area of said hub, corresponding to the parameter representation of an oval x=b/2×cos }; y=a/2×sin}for 0o==180o, and by providing it with a circular cylindrical shape in the equatorial-nadir-equatorial region of the hub, whereby a: large oval diameter; b=D: small oval diameter; D: diameter of the cylindrical hub; : the angle forming any particular unlimited line counter to the z axis.

The invention relates to a piston pin bearing for an internal combustionengine, having pin bores in which a piston pin is mounted.

It is known, for example, from DE 21 52 462 B2, DE 41 41 279 A1, and DE30 36 062 C2 to configure pin bores of pistons for internal combustionengines so as to meet stress and deformation requirements. The shapes ofthe pin boss that are mentioned there result from the generalrecognition that the gas forces that act on the piston crown aretransferred to the piston crown by way of the pin bosses, whereby thepiston pin is periodically bent through as a result of its rotationalmovements in the connecting rod. According to the conventionalassumption, the pin bores are thereby stressed both in the horizontalplane and the vertical plane, particularly, however; at the zenith andnadir of the pin bore, with regard to tension, pressure, and bending. Inorder to meet the requirements of this deformation, DE 21 52 462 B2proposes a boss shape in which the mantle line of the pin bore is bent,the axis of the pin bore assumes a slightly curved progression towardsthe center of the piston, and the cross-section of the bore is shaped inoval shape, whereby the small semi-axis of the oval runs parallel to thelongitudinal axis of the piston.

In DE 30 36 062 C2, it is proposed that the pin bores are configured tobe oval in cross-section and that the great semi-axis of the oval runsparallel to the longitudinal axis of the piston. In addition, the sideof the pin bores removed from the longitudinal axis of the piston has agreater ovality, and the side adjacent to the longitudinal axis of thepiston has a smaller ovality, whereby in another embodiment, the mantleline is configured in an incline at the apex of the bore.

All of the aforementioned shapes, however, do not prevent cracks fromoccurring in the skirt and in the region of the bowl edge, as well asthe bowl base, in practice, with an increasing stress on the piston dueto increasing ignition pressures, after a few hundred hours ofoperation, the cause of which can lie in overly great play between thepiston pin and the pin bore in the zenith and the nadir of the bore. Inorder to achieve as little play as possible, an oval pin bore isproposed in DE 16 50 206 A1, in which the large axis of the oval comesto lie crosswise to the longitudinal axis of the piston. With this, thereliability against seizing and a low level of noise between the pistonpin and the pin bore are supposed to be achieved, at the same time.

It is the task of the invention to indicate an improved pin bore shapefor a piston pin bearing, as compared with the state of the art, whichallows a clear reduction in mechanical stresses in the piston andtherefore also an extension of the useful life of the piston. Inaddition, noise formation in the piston pin bearing is supposed to beavoided by the shape.

According to the invention, the task is accomplished by means of thecharacteristics of claim 1.

By means of the unilateral high ovality, which is implemented only inthe equator-zenith-equator pin boss region, the result is advantageouslyachieved, on the one hand, that in the case of the implementation of thehighly oval bore, according to the invention, the regions that liefarther outside in the pin bore, in other words the lateral regions ofthe pin bore, are subjected to greater stress. This results in a greatermoment, because of the enlarged lever arm with reference to the pinaxis, thereby reducing the bending of the piston about the pin axis.This results in lower tangential stresses at the bowl edge and the bowlbase of the combustion bowl, as compared with the state of the art.

On the other hand, because of the circular cylindrical shape that isimplemented on one side, only in the equator-nadir-equator pin bossregion, in which the mantle line of the cylinder that lies in the nadirof the pin bore runs parallel to the pin bore axis, the result isadvantageously achieved that the play between the piston pin and the pinboss is minimized in the case of a contact change of the piston pin fromthe top to the bottom of the pin boss.

In total, a clear increase in the useful lifetime of the piston isachieved by means of the shaping of the pin bore according to theinvention, as compared with the known state of the art.

Advantageous further developments are the object of the dependentclaims.

The invention will be explained in greater detail below, using anexemplary embodiment. The drawing shows:

FIG. 1 a partial longitudinal cross-section through a piston;

FIG. 2 a cross-section through the pin bore, cut along the line yy;

FIG. 3 a piston in a perspective view, with a representation of varioushighly stressed points A-E;

FIG. 4 a graphic representation of the influence of various pin boreversions on the useful lifetime, according to the positions shown inFIG. 3;

FIG. 5.1 a representation of the radial stresses in the pin bore as thedevelopment of a cylindrical pin boss shape, between theequator-zenith-equator, with a circular cylindrical pin boss shapebetween the equator-nadir-equator;

FIG. 5.2 a representation of the radial stresses in the pin bore as thedevelopment of a highly oval pin boss shape, between theequator-zenith-equator, with a circular cylindrical pin boss shapebetween the equator-nadir-equator;

FIG. 5.3 a representation of the differences in the stresses with ahighly oval and a cylindrical pin bore;

FIG. 6 a representation of the change in useful lifetime.

As is evident from FIG. 1, a piston 1 for an internal combustion enginehas a pin bore 2 having an upper pin boss region 2.1 and a lower pinboss region 2.2, in which a circular cylindrical piston pin (not shown)is mounted. According to FIGS. 1 and 2, the upper pin boss region isdisposed on the gas force side, on which the mantle surface, designatedas 5, of the pin bore 2 is shaped in highly oval shape on thecircumference, along the entire longitudinal pin boss axis X, in theregion of the pin boss equator-pin boss zenith-pin boss equator, and theparameter representation of the oval followsx=b/2×cos α}y=a/2×sin α} for 0°≦α≦180°

In the lower region 2.2 of the pin bore 2, the mantle surface 5 isconfigured in circular cylindrical shape along the entire longitudinalpin boss axis X, between the pin boss equator-pin boss nadir-pin bossequator, whereby the following applies for both pin boss regions:

-   -   a: great oval diameter;    -   b=D: small oval diameter;    -   D: diameter of the cylindrical pin boss part;    -   α: angle that any desired ray forms relative to the Z axis.

For the entire pin bore, it holds true that the mantle lines 3 and 3,which lie in the zenith and the nadir, run parallel to the longitudinalpin boss axis, which stands perpendicular to the longitudinal pistonaxis. It is practical if the ovality amounts to 0.05 to 0.2% of the pinbore diameter D.

As is evident from FIGS. 3 and 4, a dramatic increase in useful lifetimecan be achieved by means of the pin boss design according to theinvention; this results from a 1000 hour piston stress test for alight-metal diesel piston at 190 bar ignition pressure. In FIG. 3, thepiston is shown in an exploded view, with the skirt 6, connecting rod 7,and piston pin 8, as well as the measurement points A-E.

FIGS. 5.1 and 5.2 show developments of the upper pin boss region 2.1,showing the surface pressure distributions in the cylindrical and highlyoval version, as well as the difference in the pressure distributionsfrom FIG. 5.1 and 5.2, according to FIG. 5.3, when using a light metaldiesel piston, at an ignition pressure of 175 bar. As is evident, theradial stresses are higher in the outer regions of the pin bore, so thatbecause of the enlarged lever arm with reference to the pin axis, agreater moment results, which reduces the bending of the piston aboutthe pin axis. This results in reduced tangential stresses at the bowledge, for example, but also other regions in the combustion bowl areplaced under less stress.

FIG. 6 indicates the change in useful lifetime, proceeding from acylindrical pin bore that corresponds to the value of 100%. With this,the aforementioned reductions in stress, particularly at the bowl base,the bowl edge in the pin direction (MuRaBoRi) and the coolingchannel/bowl (KüKa) result in a strong increase in the useful lifetimeof the pistons.

REFERENCE SYMBOLS

-   -   1 piston    -   2 pin bore    -   2.1 upper pin boss region (highly oval)    -   2.2 lower pin boss region (circular cylindrical)    -   3 zenith, mantle line in the zenith    -   4 nadir, mantle line in the nadir    -   5 mantle surface of the entire pin bore    -   6 piston skirt    -   7 connecting rod    -   8 piston pin    -   X longitudinal pin boss axis, equator    -   Y longitudinal piston axis    -   Z crosswise pin boss axis    -   Y′ difference between the great semi-axis of the oval and the        circular diameter of the lower-pin boss-region    -   GDS counter-pressure side of the piston    -   DS pressure side of the piston

1. Piston pin bearing for an internal combustion engine, having pin bores in which a piston pin is mounted, wherein the mantle surface (5) of the pin bore (2) is shaped in highly oval shape along the entire supporting pin length, in the region of the pin boss equator-pin boss zenith-pin boss equator, and the parameter representation of the oval follows: x=b/s×cos α} y=a/2×sin α} for 0°≦α≦180° and is configured in circular cylindrical shape in the region of the pin boss equator-pin boss nadir-pin boss equator, where: a: great oval diameter; b=D: small oval diameter; D: diameter of the cylindrical pin boss part; α: angle that any desired ray forms relative to the x axis.
 2. Piston (1) for an internal combustion engine, as recited in claim 1, wherein the mantle lines of the pin bore (2) that lie in the zenith (3) and the nadir (4) run parallel to the pin bore axis.
 3. Piston (1) for an internal combustion engine, as recited in claim 1, wherein the piston pin (8) is shaped cylindrically.
 4. Piston (1) for an internal combustion engine as recited in claim 3, wherein the piston pin (8) has mold undercuts. 